1. Field of the Invention
The present invention relates to a posterior spinal osteosynthesis device intended to ensure a transverse connection between two vertebral rods extending along a spinal segment.
Such a device is of the type comprising a pair of hooks bearing laterally on a vertebra, and fixed to a means for connecting them to each other.
2. Description of the Related Art
International patent application WO 97/25931 describes a posterior transverse connection device in which two hooks are connected via a tubular element passing through a central piece placed above the spinous process of the vertebra, this central piece serving as a bearing for longitudinal rods.
When orthopedic surgeons wish to correct scoliosis, they can use pedicular screws in the area of the lumbar vertebrae by virtue of the fact that the dimensions of the pedicles of these vertebrae are sufficient for screws. By contrast, in the area of the thoracic vertebrae which are thinner than the lumbar vertebrae, the space available is rarely sufficient for screws, and this forces the surgeons to use pedicular hooks connected to transverse hooks.
When the scoliosis is pronounced, the surgeon experiences difficulties in displacing the vertebra to the desired position with the aid of hooks. This is because the vertebra must be displaced in translation in the frontal plane and in rotation in the sagittal plane by pivoting on itself. This results in a risk of sliding of the hooks on which the surgeon is exerting pressure. In brief, if the surgeon only has to exert a restoring force on the vertebra, a single hook may suffice. By contrast, if the vertebra must additionally turn about itself, the surgeon places a second transverse pedicular hook on the other side of the pedicle, with a connection rod passing through the two hooks, which reduces the risk of these sliding. The fact remains, however, that this risk exists because of the insufficient hold provided by the hooks.
Moreover, the hooks are adjusted as best as possible on the vertebrae without being fixed thereto. This results in an appreciable play and therefore poor bone anchoring, and this means that the restoring couple must not be too high. Furthermore, the positioning of the hooks and of the screws takes a long time and disadvantageously prolongs the time needed for the surgical intervention.
Finally, the various pedicular hooks, both transverse and laminar, are not effective in the maneuvers of rotational correction of the position of the vertebrae, their action on these vertebrae being limited to displacements in the postero-anterior or antero-posterior directions. In addition, the blade of the laminar hooks is in contact with the dura mater in the medullary canal.
The vertebral screws of the known instrument systems also present a not inconsiderable risk of touching the spinal cord when they are being fitted by the surgeon, besides the fact that they require pedicles of sufficient size.
The object of the invention is to make available a posterior spinal osteosynthesis device of the type mentioned above which is made in such a way as to permit a solid hold of the hooks on the vertebra, sufficient to eliminate any risk of sliding or tilting of the hooks during a movement of rotation imparted to the vertebra in order to correct a scoliosis.
According to the invention, the body of the hooks is designed to bear on transverse processes of the vertebra which have been resected beforehand, and the connection means consists of a pair of parallel rods which are joined together and bent elastically to form a transverse arc whose ends engage in bores formed in the hooks, and means are provided for fixing the hooks to the ends of the rods of the transverse arc in positions generating elastic restoring couples via the vertebral arc of the hooks bearing on the transverse processes.
To prepare for fitting such a device, the surgeon resects two transverse processes (or apophyses) at the appropriate site, which frees an accessible space between the resected face of each transverse process and the corresponding side when the vertebra is a thoracic vertebra. Although the first application of the device according to the invention is intended for the thoracic vertebrae, this device can also be fitted equally well on the other vertebrae, in particular the lumbar vertebrae.
Once the two hooks are in place on the transverse arc, which is bent elastically, the surgeon then uses a suitable clamp to tighten the two hooks bearing on the transverse processes, in order to slide them on the transverse arc and move them toward each other, which accentuates the curvature of the rods of the arc, which remains in its elastic range. The hooks are then locked on the arc. The latter then exerts on the hooks a restoring couple which keeps them firmly applied on the transverse processes. This thus affords a solid assembly in which the hooks do not slip on the vertebra under the forces which are imparted to them by the surgeon in order to pivot the vertebra, and they subsequently remain in place without play.
The vertebral arc advantageously consists of two welded cylindrical rods instead of one. This is because the use of a single rod would involve a risk of rotation of one hook on the rod relative to the other, whereas two parallel and integral rods prevent any tilting of the hooks on the transverse arc.
The bodies of the hooks are of whatever form but preferably delimit a U-shaped channel adapted to receive the vertebral rod and are provided with suitable means for locking the rod on the hook.
The device according to the invention permits arthrodesis of two or more vertebrae using at least two connection devices of this type connected to two vertebral rods.
According to one characteristic of the invention, the bores of the hooks have an oblong cross section complementing that of the cylindrical rods of the transverse arc.
According to one embodiment of the invention, each hook comprises a body designed to receive the corresponding vertebral rod, and a lateral stem adapted to be able to bear on the transverse process, and the oblong bore is formed at the junction between the body and the stem so as to be traversed by the end of the transverse arc; a threaded hole formed in the stem opens into the oblong hole, this threaded hole being adapted to be equipped with a screw for blocking the hook on the transverse arc.
The two rods of the transverse arc can be joined together by any suitable means, for example by laser welding.